Sitka spruce

Geographical variation in crown density for oak between 2000 and 2005. White circles show the locations of plots, and figures within the circles are mean crown densities (%). Some plots are too close to be distinguished individually. The value assigned to each 20 km square was calculated from weighted averages (weight is proportional to 1/d², where d = distance) for all plots within 70 km of the 20 km square centre.